Conventionally, a four-wheel-drive vehicle includes a drive power distribution device for distributing drive power generated in a driving source such as an engine to a main driving wheel and an auxiliary driving wheel. In this kind of four-wheel-drive vehicle, if a front wheel is the main driving wheel and a rear wheel is the auxiliary driving wheel, for instance, the drive power generated in the driving source is transmitted to the front wheel via a front drive shaft and a front differential, and to the drive power distribution device having a multiple disc clutch via a propeller shaft. And, a supply of hydraulic fluid at a predetermined pressure to the drive power distribution device from the hydraulic control device controls a engagement pressure of the drive power distribution device. Thus, the drive power of the driving source is transmitted to the rear wheel at a predetermined distribution ratio.
As a hydraulic control device for controlling an oil pressure supplied to the drive power distribution device, a hydraulic control device disclosed in Patent Document 1 is available conventionally. The hydraulic control device disclosed in Patent Document 1 is a hydraulic pressure sealing-type hydraulic control device using an electric oil pump and a solenoid valve (on-off valve). This hydraulic control device is configured to drive the electric oil pump and simultaneously close the solenoid valve, thereby engaging the hydraulic clutch that transmits torque between the front and rear wheels, and shifting a drive state of the vehicle from a two-wheel drive state to a four-wheel-drive state. Once the drive state has shifted to the four-wheel-drive state, the hydraulic pressure of the hydraulic clutch is maintained as long as the solenoid valve is closed. Therefore, without continuing to operate a motor for the electric oil pump, the four-wheel-drive state can continue, which is mentioned as an advantage of the hydraulic pressure sealing-type.